Abstract: In order to study the mechanism of the influence of exogenous optimal iron-nickel ions combination on the gas production process of fermentation, taking Daliuta lignite as an example, experiments were carried out with different contents of iron-nickel ions and the optimal concentration of single ions was elected. The optimal combination concentration of iron-nickel ions were determined by combined design of quadratic regression and common rotation. The fermentation broth of experiment group and blank group(not to added any ions), the coal sample of before and after experiment were analyzed by gas chromatography, X-ray diffraction, Fourier transform infrared spectroscopy, etc. The results showed that the optimum concentrations of Fe²⁺ and Ni²⁺ were 15 mg/L and 0.005 mg/L, respectively, and the addition of Ni²⁺ showed an obvious trend of promotion at low concentration and suppression at high concentration. The gas production of experiment group increased by 13.64% and 20.69%, respectively, compared with blank group. The peak time of pH, VFA and OD₆₀₀ of experiment group was ahead of 3 d, and the macromolecular structure of the coal was more fully degraded. Finally, the effect of iron-nickel ions combination on methanogenic process was analyzed and the internal mechanism of iron-nickel ions in promoted methane production by coal fermentation was clarified, which laid a theoretical foundation for improving the yield and efficiency of secondary biogenic gas.